Common-Mode Current (CMC) Issues in EFHWs vs Off-Center Fed Antennas
The End-Fed Half-Wave (EFHW) and the Off-Center Fed Dipole (OCFD) are both popular multiband wire antennas. They promise easy coverage of several bands from one feedpoint — but their common-mode current (CMC) behavior is very different. And that difference is critical if you have neighbors close by.
Why Off-Centers Usually Handle CMC Better
An OCFD fed at an offset point sees a more moderate impedance than an EFHW’s high-voltage end. This makes it easier to transform and less likely to excite the coax shield as part of the antenna. With a properly placed 1:1 current choke, OCFDs generally push less RF onto the feedline than EFHWs.
The Wideband EFHW “8010” Problem
The classic EFHW was originally a monoband antenna, later adapted for simple dual-band use. In that role it worked — with the right transformer and choke placement. But marketeers pushed it further into the so-called “8010” multiband EFHW, claiming 80–10 m coverage with no tuner. In practice, this “trickster” relies on the coax feedline as part of the radiator, which leads to unpredictable SWR, shack RF, and neighbor RFI.
- EFHW (classic, mono/dual band, no choke): ≈0–10 dB CMC rejection — feedline “hot,” but manageable with a choke at ≈0.05 λ.
- EFHW “8010” (wideband marketed): very high CMC — needs ≥30 dB chokes at 0.05λ, shack entry, and rig.
- OCF (optimized) + 1:1 choke: ≈10–20 dB CMC reduction.
- EFOC (engineered OCF) + two chokes: ≈20–35 dB CMC reduction.
- TermiLoop (terminated loop): ≈25–35 dB CMC reduction, very stable.
Disclaimer — Indicative only. Actual results depend on height, soil, feedline length, and nearby structures. The takeaway: without proper choking, these antennas excite their coax shields.
Comparison at a Glance
| Antenna Type | Band Coverage | CMC Tendency | Choke Requirement | Neighbor-Friendliness |
|---|---|---|---|---|
| EFHW (classic) | Monoband or simple dual-band | Moderate–high | ≥30 dB choke at 0.05λ + shack | Usable with proper choking |
| EFHW “8010” (wideband) | Marketed as 80–10 m no tuner | Very high — coax radiates | ≥30 dB chokes at 0.05λ, shack, rig | Poor without mitigation |
| OCF Dipole | Dual-band (e.g., 40/20) + harmonics | Moderate | 1:1 choke at feedpoint or 0.05λ | Better than EFHW |
| EFOC Series | Engineered multi-band | Low–moderate | Two chokes (0.05λ + shack) | Good choice for suburban lots |
| TermiLoop | Broadband terminated loop | Very low | One choke at shack entry | Excellent — least RFI risk |
If You Keep the “Marketing Trickster,” Do This
- Use at least 30 dB effective choking across 1–30 MHz.
- Install one choke at ≈0.05 λ from the feedpoint (based on lowest band).
- Add another choke before shack entry.
- Add a third near the rig for audio/USB hygiene.
Our 30 dB Wideband HF Choke (1–30 MHz) is purpose-built for exactly this — keeping your feedline a feedline.
- Use Type 31 ferrite for wide HF coverage (1–30 MHz).
- FT240-class cores are the safe baseline — stack 2–3 for higher power.
- Wind 12–14 turns of RG-316/174 for QRP, or RG-400/mini-8 for 100 W+.
- Aim for ≥30 dB choking on your lowest band of use.
- Avoid FT140 cores — they saturate quickly, heat up, and rarely deliver enough choking impedance on HF.
For guaranteed 1–30 MHz suppression and full power handling, our ready-made choke saves trial-and-error.
Mini-FAQ
- Why do EFHWs cause more CMC? — The end feedpoint is high-Z and voltage-driven, which easily couples imbalance into the coax.
- Are OCFDs truly balanced? — No, but their offset feedpoint is less extreme and works better with a choke.
- What tames the 80–10 “wideband” EFHW? — At least two ≥30 dB chokes (0.05λ + shack entry) and ideally a third at the rig.
- Best neighbor-friendly choice? — TermiLoop or EFOC with correct choking.
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